Osteotomies are currently performed with mechanical tools such as oscillating saws, chisels or drills. The precision in the cut, drilling or milling in the bones and cartilages obtained with these tools is limited by the size of the instrument used and only simple cutting geometries can be performed with these tools. An inherent drawback of using mechanical tools for osteotomy is that they are in direct contact with the hard tissue transmitting unwanted vibrations to the patient and, the heat generated by frictions, degrades the otherwise obtainable precision in the osteotomy.
Taking advantage of laser ablation methods of wide use in the micromachining of non-biological materials such as metals and plastics for replication and fast prototyping a new method to perform contact-free osteotomies is emerging offering distinct advantages over mechanical methods (see e.g. Kuttenberger J J., Stübinger S., Waibel A., Werner M., Klasing M., Ivanenko M., Hering P., Von Rechenberg B., Sader R., and Zeilhofer H F., Photomed Laser surg., 2008 April; 26(2):129-36 and references herein). However, an important difference is encountered when micromachining biological tissues of patients by photoablation, as compared with e.g. metals or plastics, which contributed to delays in its development for osteotomic purposes, is the difficulty to properly fix the anatomical target of the patient to be operated. This difficulty precludes that the precision of the intervention be dominated by the size of the beam waist (the size of the laser beam at its focal point) but by the movements and vibrations of the invention overcomes such difficulty in the preferred embodiment by the use of a novel auto-tracking correcting tool which constantly compensate for the movements and vibrations of the anatomical part being operated.
Another possible reason accounting for delays in the implementation of laser micromachining of biological tissues is that laser micromachining of non-biological materials was justified from a commercial perspective because it is primarily used for the replication of parts (e.g. as in the auto industry) whereas in osteotomy every intervention is unique. However, modern imaging techniques of hard and soft tissues combined with fast prototyping methods for pre-operative planning, as used in the present invention, facilitates the individual design step justifying now the use of laser micromachining for individual cases. Moreover, the use of these modern techniques becomes imperative for complicated cases.
The interaction of laser light with hard tissues, which is the first step in order to obtain efficient photoablation, has been studied in great detail using various types of lasers as shown in various clinical studies. In biological context, the term “photoablation” and derivations thereof as used herein refers to the vaporization of water in human tissues and its subsequent ejection induced by pulsed laser irradiation of selected wavelengths, specific powers and pulse durations. The deposited electromagnetic energy is almost entirely transformed into mechanical energy and the illuminated region is ejected at high velocity in the form of debris. The deposited energy is thus removed by the ejected debris precluding or minimizing the dissipation of heat minimizing thus thermal damage into the surface of the remaining tissue of significant relevance for the healing process.
Bone materials consist approximately of 13% water, 27% collagen and 60% hydroxyapatite and calcium phosphate. The mineral component of bone material is found in the form of hydroxyapatite crystallites, which is a form of calcium phosphate. The crystallites are surrounded by amorphous calcium phosphate and embedded in a collagen matrix. They reach a maximum size of 50 nm and are clustered along the collagen fibrils in distances of 60-70 nm; the clusters size up to a few micrometers. The melting point of the minerals is about 1500° C. Because the spectral characteristics of bone tissues are dominated by the absorption spectrum of water, the lasers that are known to efficiently photoablate bone and cartilage tissue are CO2 gas lasers lasing at 10.6 μm, solid state Erbium lasers lasing at wavelengths of 2.94 μm and 2.79 μm (depending on the type of gain media), Holmium lasers lasing at 2.08 μm, Excimer lasers lasing at wavelengths shorter than 300 nm (Yow L., Nelson J. S. and Berns M. W., Laser Surg., Med.; 1989, 9, 141-147) solid state lasers Q-switched lasers with pulse widths of a few nanoseconds of various emitting wavelengths and, ultrafast femtosecond lasers (e.g. Girard B., Cloutier M., Wilson D J., Clokie C M., Wilson B V., Laser. Surg. Med., 2007 June; 39(5): 458-67). The two most used lasers are however pulsed CO2 and Erbium lasers. Erbium lasers have shown some advantages (see e.g. Stübinger S., Nuss K., Landes C., von Rechenberg B., and Sader R. Laser Surg. Med., 2008 July; 40(5):312-8) over CO2 lasers in terms of cutting precision and thermal damage. The higher precision observed in cuttings performed by Er:YAG (Erbium:yttrium aluminum garnet) lasers is primarily due to the fact that the absorption coefficient of liquid water is higher at 2.94 μm (of 12×103 cm−1) than at 10.6 μm (of 0.7×103 cm−1) resulting in short optical penetration by explosive vaporization. With pulses from free-running Er:YAG lasers this photoablation process is very efficient resulting in substantial ablation yields of 0.6 mm3/J with minimal thermal damage of about 10-15 μm in depth. In contrast, CO2 lasers remove bone tissue by heating it up to the vaporization point and pyrolysis resulting in extensive char formation (i.e. carbonization) translating in delayed healing. Erbium lasers are thus more appropriate than CO2 lasers and are used in the preferred embodiment of the present invention.
Laser cuttings have been done by directing the laser beam using e.g. fiber optical tips (Stübinger S., Landes C., Seitz O., and Sader R.; J. Periodontol. 2007 December, 78(12):2389-94), with the help of laser beam manipulators controlled either by a joystick operated by the surgeon or, by means of a computer controller where the predefined cutting path such as the split line in a craniomaxillofacial (CMF) surgery, referred herein as the osteotomy line, has been previously programmed while the surgeon monitors the intervention. The manipulators used in the most advanced devices are based on XY deflecting mirrors (Kuttenberg J J., Stübinger S., Waibel A., Werner M., Klasing M., Ivanenko M., Hering M., Von Rechenber B., Sader R., Zeilhofer H F.; 2008 April; 26(2): 129:36). Sophisticated cuts have been achieved by mounting the beam focusing elements in this computer controlled optical scanner. Such optical delivery system is however best suitable to cut relatively flat bones because the beam scanner is fix with respect to the anatomical target and the beam waist cannot follow the complex curved bone tomography as those encountered in e.g. CMF surgery. Another problem associated with the use of manipulators based on XY moving mirrors is that the scanner is fixed and the distance from the focusing lens to the photoablation spots changes along the osteotomy line; i.e. it does not account for changes in the Z axis (coaxial to laser beam). This problem becomes serious when the osteotomy line is relatively large when using XY mirror scanners because the precision varies along the osteotomy line; i.e. it degrades when the beam waist is not at the target. It is thus desirable to have a means to ensure that the beam waist is always positioned at the spot to be photoablated. Besides the lack of control of the Z axis these XY beam deflectors do not allow for the control of the striking angle of the laser beam, defined by the angles ⊖ and Ω with respect to the tissue to be photoablated, required to augment the complexity in the cutting geometries or, required to avoid features such as a nerve or a tooth. In the preferred embodiment of the present invention the laser head of a compact solid state laser is mounted into the last segment of a robot-guided arm having several degrees of freedom which is capable of positioning the laser beam waist along the entire osteotomy line at any convenient striking angle. In another embodiment of the invention, this problem is solved by using an automatic autofocus system which ensures that the beam waist always lies on the desired place of the target, e.g. on the surface of the bone.
The precision of osteotomy has been greatly improved by the use of Operative Planning and Surgical Navigation methods from Computer Assisted Surgery (CAS) used to perform several types of complex interventions. Very important to CMF osteotomy is the possibility to have prior to the intervention a 3D representation of the anatomical region to be operated obtained nowadays by modern scanning technologies. These scannings are nowadays done by a number of available medical imaging technologies including CT (Computer Tomography), MRI (Magnetic Resonance Imaging), X-Rays, Ultrasound etc. Furthermore, different scanning methods can also be combined to obtain the final 3D dataset using fusion techniques. The final 3D dataset reproduces the exact geometrical situation of the normal and pathological tissues or particular structures of the region of interest. Artificial color contrast of the 3D dataset provides details of e.g. soft vs. hard tissue structures, allowing thus a computer to differentiate, and visually separate, different tissues and structures and, to prevent vulnerable anatomical structures from damage.
The 3D dataset reproducing the anatomical region of interest often includes intentional landmark features which are useful to realign the virtual dataset against the actual anatomy during surgery for navigation purposes. Different surgical guides, or headframes, to be attached to the patient's head for oral and CMF osteotomies have been developed. Optical positioning systems based on an infrared (IR) camera and various transmitters attached to e.g. the skull for CMF interventions offer distinct advantages to mechanical surgical guides. These are also positioned in convenient regions of the neurocranium and, in some cases, also in the instrument used to cut the bone. At least three IR transmitters attached in the neurocranium area are needed to compensate for the movements of the patient's head. In practice more than five transmitters are used to improve the precision, more preferentially more than seven transmitters are convenient. The 3D position of each transmitter is measured by the IR camera, using the same principle as used in satellite navigation. The workstation of the surgical navigator is constantly visualizing the actual position of the free-moving bone structures or fragments which are compared with the predetermined target position. In this way the bony fragments from the osteotomy can be accurately positioned into the target position a-priori determined by surgical simulation. In the preferred embodiment of the present invention an optical positioning navigation system is used in such way that its output feeds a novel auto-tracking correcting system connected to the robot-guided arm constantly compensates for the movements and vibrations in the anatomical part being operated to ensure that the actual cutting accurately corresponds to the predetermined target osteotomic line.
In another embodiment, a set of IR emitters is attached to the robot-guided arm where the photoablation laser is mounted to enhance the precision and safety of the instrument.
Summarizing, in contrast to mechanical methods, state-of-the-art photoablation methods offers the possibility to perform osteotomies with the following advantages:    a) Non-contact almost vibrationless interventions.    b) Higher precision.    c) Decreased bleeding.    d) Reduced postoperative period due to faster healing.    e) Possibility to perform complex cutting geometries.    f) Easy combination with available Surgical Navigation methods and Operative Planning.    g) Constant control the depth in the cutting, drilling or grinding of bone and cartilage to minimize or completely avoid damage in vulnerable structures (e.g. vessels and nerves) and the surrounding soft tissue
However, there are a few remaining recognized issues which precludes its wide use to benefit a large segment of patients. Some of the drawbacks in osteotomies performed with state-of-the-art devices are:    a) Lack of an auto-tracking method to lock the coordinate system of the osteotomy line to the anatomical part of the patient being operated which is independent of the movements of the patient to precisely perform a pre-defined cutting path.    b) The limited degree of freedom in the manipulation systems to properly direct the photoablation laser beam to the osteotomy line.    c) The slow speed in the manipulation of the laser beam waist over the osteotomy line.    d) The lack of a system capable of removing photoablation gases and particles causing odor and distributing pathogenic particles into the operating theater.    e) Lack of safety features to stop the photoablation process by the surgeon in eventual emergency situations.
There is thus a need for a CARLO medical device which addresses the above mentioned deficiencies capable of faster and safely performs a predefined cutting geometry at various traversing angles at a constant precision which is independent from the movements of the patients and evacuates the photoablation debris and odor.